Photorefractive device is a nonlinear optical device based on the photorefractive effect of material. It may be a piece of photorefractive crystal processed by certain technique, or an optoelectronic assembly including at least one piece of photorefractive crystal as the core part.
The fundamentals of the photorefractive device can be found in many public materials such as "Photorefractive Materials and Their Applications I and II" edited by P. Gunter and J.-P. Huignard, Topics Appl. Phys. Vol. 61 and 62 (Springer-Verlag, Berlin, Heidelberg, 1988 and 1989).
One group of most important photorefractive materials is the ferroelectric oxide crystal, among which tetragonal-phase BaTiO.sub.3 has the best photorefractive properties. The particular advantage of BaTiO.sub.3 is that the photorefractive effect is very large and it can produce a phase conjugate beam using self-pumping mechanism. One of the main disadvantages of BaTiO.sub.3 crystal is to have the difficulty in obtaining of single crystal which has desirable size and optical quality due to the existence of a hexagonal phase above 1460.degree. C. Single crystals of BaTiO.sub.3 are normally grown by TSSG technique, of which the growth period is typically 20.about.30 days, the equipment and technological requirements are complicated, and yield is low.
BA .sub.1-x Sr.sub.x TiO.sub.3 is a mixed oxide crystal. In the region of x&lt;0.3, Ba.sub.1-x Sr.sub.x TiO.sub.3 crystal is tetragonal. ferroelectric phase (space group P.sub.4 mm-C.sup.1.sub.4.nu.) at room temperature. Upon heating, it transforms to a cubic, paraelectric phase (space group Pm.sub.3 m-O.sup.t.sub.h) at the Curie temperature Tc. The value of Tc depends on the value of x.
The phase diagram of BaTiO.sub.3 --SrTiO.sub.3 system has been reported by J. A. Basmajian and R. C. DeVries in Journal of the American Ceramic Society, Vol. 40, P. 373 (1957). They found that the BaTiO --SrTiO system is characterized by a complete series of solid solutions and the hexagonal phase is suppressed to a region extending no farther than 0.5% SrTiO.sub.3. Thus, for x&gt;0.005, Ba.sub.1-x Sr.sub.x TiO.sub.3 can be grown from the melt. Since then, several efforts have been reported in literature to grow Ba.sub.1-x Sr.sub.x TiO.sub.3 single crystals with different value of x by czochralski method or solvent zone melting technique (e.g. D. Rytz et al. Japanese Journal of Applied Physics, Vol. 24 supplement 24-2, P. 622 (1985); R. M. Henson and A. J. Pointon, Journal of Crystal Growth vol. 26, P. 174 (1974)). So far, there has been no report on obtaining of Ba.sub.1-x Sr.sub.x TiO.sub.3 single crystal suitable for measurement of their photorefractive properties. And also no one has reported that Ba.sub.1-x Sr.sub.x TiO.sub.3 crystal possesses good photorefractive properties or can be used to construct a photorefractive device.